Touch screens allow a user to conveniently interface with an electronic display system. For example, a user can carry out a complicated sequence of instructions by simply touching the screen at a location identified by a pre-programmed icon. The on-screen menu may be changed by re-programming the supporting software according to the application.
Resistive and capacitive are two common touch sensing employed to detect the location of a touch input. Resistive technology typically incorporates two transparent conductive films as part of an electronic circuit that detects the location of a touch. Capacitive technology, on the other hand, typically uses a single transparent conductive film to detect the location of an applied touch.
A touch location is generally determined by applying an electric field to a transparent conductive surface in the touch area. Where the transparent conductor is an electrically continuous coating in the touch area, the accuracy of detecting the location of an applied touch depends on the linearity of the electric field in the transparent conductor.
Various methods have been proposed to linearize the electric field. For example, in a four wire resistive touch technology, a pair of highly conductive continuous electrode bars are formed onto a transparent conductive surface at two opposite edges of a touch surface. A differential voltage applied to the two conductive bars results in a fairly linear electric field in the plane of the transparent conductive surface in the direction normal to the two electrode bars. Similarly, a second pair of highly conductive electrode bars are formed on a second conductive surface with the bars being orthogonal to the first pair of bars.
As another example, five wire resistive or capacitive touch sensors employ an electrode pattern that may be formed on a transparent conductive surface along the perimeter of a touch area to linearize the field. In a five wire resistive touch sensor, a second transparent conductor can act as a current sink or voltage probe and may not require linearization. In a five wire capacitive touch sensor, a user's finger or other conductive implement may provide the current sink. The electrode pattern is typically made up of a number of discrete conductive segments positioned in such a way as to generate a linear orthogonal field in the plane of the transparent conductor.
Typically, the linearizing electrode pattern includes several rows of discrete conductive segments positioned along the perimeter of a touch area, such as disclosed in U.S. Pat. Nos. 4,198,539; 4,293,734; and 4,371,746. The conductive segments are typically electrically connected to each other via the conductive surface they are deposited IS on. U.S. Pat. No. 4,822,957 discloses rows of discrete electrodes having varying lengths and spacings to linearize the electric field in a touch area.
Several factors can determine the efficacy of a linearization pattern. One such factor is the degree to which the field can be linearized. Some electrode patterns may be incapable of linearizing the field to a level required in a given application. Another factor is the overall width of the electrode pattern. Linearity of the electric field can, in general, be improved by increasing the number of rows of electrodes. Increasing the number of rows, however, tends to increase the touch panel border. This may be so because the electrode pattern is typically made of highly conductive opaque materials, such as metals, and is, therefore, placed outside the touch area as to not interfere with the viewing of displayed information. Therefore, improving field linearity may adversely affect the border size of a touch panel.
Another factor is sensitivity of field linearity to small variations in the electrode pattern. Such variations are typically unavoidable during manufacturing. If small variations in the electrode pattern result in unacceptable nonlinearity in the electric field, the yield and hence the cost of manufacturing a touch sensor may be adversely affected. Known linearization patterns may be limited by how effective they are in linearizing the electric field, and/or they may require a wider border to effectively linearize the field or compensate for dimensional errors introduced during manufacturing, and, as a result, may involve high manufacturing costs.